Investigating underlying brain structures and influence of mild and subjective cognitive impairment on dual-task performance in people with Parkinson’s disease

Cognitive impairment can affect dual-task abilities in Parkinson’s disease (PD), but it remains unclear whether this is also driven by gray matter alterations across different cognitive classifications. Therefore, we investigated associations between dual-task performance during gait and functional mobility and gray matter alterations and explored whether these associations differed according to the degree of cognitive impairment. Participants with PD were classified according to their cognitive function with 22 as mild cognitive impairment (PD-MCI), 14 as subjective cognitive impairment (PD-SCI), and 20 as normal cognition (PD-NC). Multiple regression models associated dual-task absolute and interference values of gait speed, step-time variability, and reaction time, as well as dual-task absolute and difference values for Timed Up and Go (TUG) with PD cognitive classification. We repeated these regressions including the nucleus basalis of Meynert, dorsolateral prefrontal cortex, and hippocampus. We additionally explored whole-brain regressions with dual-task measures to identify dual-task-related regions. There was a trend that cerebellar alterations were associated with worse TUG dual-task in PD-SCI, but also with higher dual-task gait speed and higher dual-task step-time variability in PD-NC. After multiple comparison corrections, no effects of interest were significant. In summary, no clear set of variables associated with dual-task performance was found that distinguished between PD cognitive classifications in our cohort. Promising but non-significant trends, in particular regarding the TUG dual-task, do however warrant further investigation in future large-scale studies.

In summary, PD-MCI performed worse compared to PD-NC on measures of dynamic balance (Mini-BESTest), functional mobility (TUG single-and dual-task), self-reported depression (HADS), and global cognition (MoCA).PD-MCI also walked with higher step-time variability during dual-task gait, and had slower reaction times during both single-and dual-task compared to PD-NC.PD-SCI had worse values compared with PD-NC in the TUG dual-task and HADS depression scale.PD-MCI had worse values compared with PD-SCI in MoCA, dual-task step-time variability, as well as dual-task gait speed, and dual-task interference on gait speed.There were no differences in age, sex, total intracranial volume (TIV), and MDS-UPDRS-III between PD cognitive classifications.See Table 1, and e-Table 1 for complete list of comparisons.

Associations between dual-task performance and PD cognitive classifications
An overview of all linear regression models can be found in Table 2 and the details of the results are described in the following.The significance level for all analyses was set to p < 0.05 and was lowered according to multiple comparisons procedure 32 .We report results that have been significant before correction as "tendencies" and results that were significant after correction as "significances".

Gait speed
Dual-task gait speed was associated with an interaction of PD cognitive classification and single-task gait speed, while sex, MDS-UPDRS-III, and age were included as additive covariates (e-Table 2).This model was significant (F(8,47) = 16.33,p < 0.001, R 2 = 0.74) with single-task gait speed (beta = 0.78, p < 0.001) being the only significant variable.In the ANOVA, single-task gait speed was significant and PD cognitive classification by single-task gait speed interaction and MDS-UPDRS-III showed a trend that did not survive the correction for multiple comparisons as a considerable number of models were trained in this study.Dual-task interference was not associated with PD cognitive classification, age, sex, and MDS-UPDRS-III (e-Table 3).

Timed up & go test
The association of TUG dual-task with PD cognitive classification, TUG single-task, sex, and age, as covariates, and the interaction of the PD cognitive classification by MDS-UPDRS-III was significant (F(6,48) = 7.58, p < 0.001, R 2 = 0.49) (e-Table 8).TUG single-task (beta = 1.09, p < 0.001) as well as the interaction of PD cognitive classification with MDS-UPDRS-III (beta = 0.38, p = 0.04) were significant variables.PD-MCI, sex, and MDS-UPDRS-III showed a trend to be associated with TUG dual-task.The ANOVA showed that TUG single-task was the only significant variable, while the interaction of PD cognitive classification with MDS-UPDRS-III, sex, and MDS-UPDRS-III showed a trend that did not survive multiple comparisons (e-Table 8).The TUG difference model was not significant (e-Table 9).

Associations between dual-task performance, PD cognitive classifications, and gray matter structures
An overview of all linear regression models can be found in Table 2 and the details of the results are described in the following.

Voxel-wise regression
Our exploratory voxel-wise regression analyses showed that dual-task gait speed correlated with the right cerebellum volume (Family-wise error correction, FWE p < 0.05, e-Table 10).This cluster was then extracted as an ROI for the linear models.The regressions with the remaining dual-task variables were not significant.

Gait speed and ROIs
All linear regression models including NBM, hippocampus, DLPFC, or cerebellum as variables were significant and single-task gait speed was always the only significant variable (e-Tables 11, 13 www.nature.com/scientificreports/For dual-task interference on gait speed, the NBM, DLPFC, and hippocampus models were non-significant (e-Tables 12, 14, 16).The cerebellum dual-task interference model was significant (F(8,46) = 2.67, p = 0.03, R 2 = 0.32) and PD-NC, although non-significant, showed a trend (e-Table 18).Regarding the ANOVAs, singletask gait speed was always the only significant variable.
The dual-task step-time variability model with the cerebellum as a variable was significant (F(9,45) = 6.52, p < 0.001, R 2 = 0.57) and single-task step-time variability a significant variable (beta = 0.77, p < 0.001) (e-Table 25).Albeit non-significant, PD-NC, cerebellum, and age showed a trend towards an association.In the ANOVA, single-task step-time variability was significant and cerebellum showed a trend.
No significant results were found for the dual-task interference on step-time variability including the NBM, DLPFC, and hippocampus (e- Tables 20, 22, 24).
The dual-task interference on step-time variability cerebellum model was significant (F(8,46) = 3.26, p = 0.03, R 2 = 0.36) (e-Table 26).PD-NC, cerebellum, and age showed a trend to be associated with dual-task interference on step-time variability.In the ANOVA, cerebellum and age showed a trend that did not survive multiple comparisons.

Timed up & go and ROIs
The model of TUG dual-task as a function of NBM was significant (F(9,45) = 5.95, p < 0.001, R 2 = 0.54) (e-Table 35), with TUG single-task as a significant variable (beta = 1.14, p = 0.001).Although statistically nonsignificant, the interaction of PD cognitive classification and NBM showed a trend towards an association.In the ANOVA, only TUG single-task was significant, while NBM showed a trend.The DLPFC model for associations with TUG dual-task was significant (F(11,43) = 6.00, p < p < 0.001, R 2 = 0.6) (Fig. 3, e-Table 37), and significant variables included TUG single-task (beta = 0.98, p = 0.002) and the interaction of PD-MCI and MDS-UPDRS-III (beta = 0.37, p = 0.005).The variables sex and MDS-UPDRS-III showed a trend to be significantly associated.
The ANOVA also highlighted a trend of significance for the interaction of PD cognitive classification by MDS-UPDRS-III.Further, only TUG single-task was significant and sex and MDS-UPDRS-III showed also a trend.
The group effect (PD cognitive classification) for PD-SCI, sex, MDS-UPDRS-III, and the interaction of PD-SCI with cerebellum showed a trend.The ANOVA highlighted a trend for the variables PD cognitive classification, PD cognitive classification by cerebellum, PD cognitive classification by MDS-UPDRS-III, sex, and MDS-UPDRS-III.Only TUG single-task was significant.
For TUG difference, the NBM model was not significant.Although NBM exerted a trend in both, the linear regression model as well as the ANOVA, it did not survive multiple comparisons.(e-Table 36).The models including DLPFC, hippocampus, and cerebellum were not significant (e-Tables 38, 40, 42  www.nature.com/scientificreports/

Voxel-based morphometry
No gray matter differences in the whole brain or when restricting with ROI masks were detected comparing PD-SCI with PD-MCI, PD-MCI with PD-NC, and PD-SCI with PD-NC using an FWE threshold cluster-level p < 0.05.Using an exploratory threshold (voxel-level p < 0.001, k = 100) whole-brain gray matter volume in PD-MCI was lower compared with HC in the right superior/middle frontal gyrus and the right superior/middle temporal gyrus (Fig. 4, e-Table 10).In the left hemisphere, we found lower gray matter volume in the superior frontal, superior/middle temporal, angular/supramarginal gyrus, and frontal pole in PD-MCI compared to controls.Higher gray matter volume in PD-MCI compared with HC was evident in the left occipital pole.PD-SCI showed lower gray matter volume compared with HC in the right hemisphere in the middle/ inferior temporal gyrus and on the left in the middle temporal gyrus and inferior temporal gyrus/ inferior occipital (Fig. 4, e-Table 10).

Discussion
This study aimed to evaluate whether cognitive classification and its structural brain correlates are associated with dual-task performance in PD.Although a majority of regression models did survive multiple corrections, certain explanatory variables in the models did not.The results suggested non-significant trends for associations between dual-task performance and GM alterations in the hippocampus, DLPFC, and cerebellum.We further hypothesized that dual-task interference on both gait speed and step-time variability would be greater in PD-MCI compared to PD-NC, but this was only partially supported by our analyses.While group comparisons were significant for both dual-task interference on gait speed and step-time variability, our posthoc tests revealed that these were driven by differences between PD-MCI and PD-SCI.Our hypothesis that dual-task performance www.nature.com/scientificreports/would be negatively correlated to GM alterations in the following manner PD-MCI > PD-SCI > PD-NC was also only partially supported.Some brain regions showed a trend towards being associated with PD-MCI (hippocampus, DLPFC, and cerebellum) or PD-SCI (cerebellum) only, but no main effect of group for all PD cognitive classifications was found.We expected that the brain regions showing a negative correlation with dualtask performance would be the hippocampus, DLPFC, and NBM.Although tendencies were found between dual-task performance with the hippocampus and DLPFC, the correlations were non-significant and not always negative.The hippocampus showed a trend toward an association between TUG dual-task and PD-MCI in which higher hippocampal alterations were linked to a longer time to complete TUG dual-task, but this was non-significant after multiple corrections.Higher DLPFC alterations were significantly associated with lower step-time variability during dual-task walking, whereas no trends or significant associations were found for NBM.
In addition, we expected atrophy in the left hippocampus, left insula, putamen, claustrum, and right putamen, as well as in the bilateral pallidum in the whole-brain analyses when comparing PD-MCI with HC.Our cohort showed no robust gray matter volume changes when PD groups were compared to each other or to HC.Although we did find atrophy in PD-MCI as compared to HC, it was found only when using a lenient threshold and in other brain regions than the ones we expected.Using this lower threshold, atrophy was also found in the bilateral middle/inferior temporal gyrus in the PD-SCI cohort and frontotemporal atrophy in the PD-MCI cohort.Lastly, www.nature.com/scientificreports/we did not expect atrophy in PD-NC compared to HC, and this was supported by our analyses which showed that even with the lower threshold, PD-NC had no atrophy.Among our pre-selected ROIs, only NBM volume differed significantly between the groups (PD-MCI-PD-SCI).In exploratory voxel-wise regressions, we identified the cerebellum as a structure associated with dual-task performance, which also significantly differed in volume between the groups.The finding of an association between dual-task performance and cerebellar alterations and how it differed according to cognitive status is novel and has to our knowledge not been reported previously in people with PD.
Although statistically non-significant in the current study, the cerebellum showed a trend towards an association with dual-task performance and interference.In our sample with mild to moderate PD, a higher cerebellar volume tended to be associated with better dual-task performance in the PD-NC group.Cerebellar involvement in dual-tasking in people with PD has been shown previously 33,34 .Results from an fMRI study indicated that the cerebellum is additionally activated during dual-task in HC compared to people with PD 33 .In that study, the cerebellum was not activated in people with PD during dual-task, but during the pure motor task, a higher activation was evident.Further, people with PD had higher connectivity between the right cerebellum and motor and cognitive networks than HC.The authors concluded that people with PD have limited cerebellar capacity, which might already be overloaded by performing a single task and thus need to integrate motor-cognitive networks to compensate.In general, cerebellar structure and function could be modulated by dopaminergic medication in people with PD.In a recent resting-state fMRI study of 24 people with PD, it has been shown that even a single dosage of levodopa significantly influences the cerebellar connectivity 35 .
Here, major connectivity increases between the cerebellum and the motor system regions such as the thalamus, putamen, globus pallidus, and brainstem have been found.
We had hypothesized that NBM structure would be associated with step-time variability but were unable to confirm this.A longitudinal study found NBM volume decreases over time to be predictive of increased steptime variability 26 , but in line with our results, the authors were unable to confirm a cross-sectional relationship.
In contrast to dual-task straight walking, it was instead the TUG dual-task, that showed tendencies to be associated with cognitive status.This is interesting as the TUG is a simple, quick, and widely used clinical test 36 .Compared to straight walking, the TUG also involves transfers (sit-to-stand and stand-to-sit) and turns, which places a higher demand on attentional resources, and especially while simultaneously performing a cognitive task.An additional factor that deserves attention when comparing the two dual-task assessments is the choice of cognitive task.Whereas a serial subtraction task was used during TUG dual-task, an auditory Stroop task was used during straight walking, and it is plausible that this may have contributed to the differing results.Interestingly, when adding ROIs to the analyses we found that the models including NBM, DLPFC, the hippocampus, and the cerebellum all showed tendencies to be associated with the time to complete the TUG dualtask.These results should however be interpreted with caution as these tendencies were non-significant.There was an interaction between hippocampal structure and PD-MCI, and between cerebellar structure and PD-SCI.For PD-MCI the direction of the effect showed that increased TUG dual-task time tended to be associated with greater hippocampal volume.For PD-SCI the effect was observed in the same direction, but here increased time to perform TUG dual-task was associated with higher cerebellar volume.This might be interpreted as a maladaptive compensatory effect on brain structure.Furthermore, as mentioned above, the cerebellum seems to be altered by levodopa treatment in PD 37 .Thus, a volume increase could be related to levodopa intake and may lead to long-term reorganization of the cerebellum that also results in structural changes.The association between increased completion time on the TUG single-task and regional brain volume reductions has been reported previously, but to the best of our knowledge, this is the first study to report an association between TUG dual-task and gray matter volume increases in a PD population 38 .
Except for dual-task step-time variability, models exploring associations between cognitive status and dualtask interference without adding brain ROIs as variables were not significant.The interference that occurs during dual-tasking has been suggested as a proxy measure for automaticity, and previous research has been able to explain small parts of its variance by for example cognition and disease severity 20,22 .Although described extensively in the literature, dual-task interference is however an exploratory variable that lacks validation and cutoff scores in the PD population.This may have consequences for any predictive ability of cognitive status or other behavioral variables, but also for our ability to interpret any significant findings.For this reason, we suggest reporting absolute values of dual-task performance controlled for single-task performance, as a complement to DTI outcomes.
This study has several limitations worth noting.Firstly, we used baseline data from an RCT (the EXPANd trial) 39 involving highly challenging balance training.Thus, there are potential biases as a result of the secondary nature of the data.People voluntarily signed up for participation, which may have biased our sample towards those more inclined to be physically active.The criteria for the RCT further excluded people with Hoehn &Yahr stages 1 and 4, i.e., both people with mild, unilateral motor symptoms, and people with severe motor symptoms.The exclusion of people in the earlier stages in particular may have limited the number of people classified as PD-SCI, and thereby also our ability to generalise our results to this group.Secondly, this study has no healthy controls to compare dual-task performance to.We acknowledge that the smaller sample size is another limitation with respect to structural MRI and linear regression results interpretation.The low sample size led to very conservative corrections for multiple comparisons, which lowered Type I error We would argue that these hypothesis tests, which were significant before correction for multiple comparisons, can be seen as tendencies and should also be considered, since our variables were carefully selected and we just might miss the statistical power to due to the low sample size.Further, one other potential limitation is that we classify the language and visuospatial domains with the items of MoCA, which is not as sensitive as other tests with better psychometric abilities.Thus, the PD-MCI group could potentially be larger.Since the data is cross-sectional, we cannot make any predictions about dual-task performance in relation to PD cognitive status.Future studies should take into www.nature.com/scientificreports/account longitudinal data including repetitive sMRI and cognitive impairment classification.Another limitation might be that we did not correct for multiple comparisons, which is debatable.Since we carefully selected variables based on the literature, we refrained from correction but want to note to interpret the results with caution.
In conclusion, no clear set of variables associated with dual-task performance was found that could distinguish between the different cognitive classifications in our sample with mild to moderate PD.However, the TUG test with an added serial subtraction task seemed to be the most promising dual-task measure, which tended to be associated with cognitive status, and with differences in brain structure.This is promising given the widespread use of the TUG dual-task in the clinic, its simple execution, and low cost.As neurorehabilitation is shifting focus from a one-size-fits-all mindset to a personalized precision rehabilitation, clinicians need to mind the cognitive heterogeneity of PD.We therefore propose future research assess a possible association between the TUG dualtask, cognitive status, and brain alterations in larger samples.

Participants
Data were collected within the framework of a randomized controlled EXPANd trial (EXercise in PArkinson's disease and Neuroplasticity) 40 .The participants underwent a broad and detailed assessment of balance/gait, overall motor impairment, neuropsychological test battery, and MRI before and after the interventions.The present study investigates a subcohort: baseline data of participants with complete gait, neuropsychological, and MRI assessments.For the EXPANd trial, a total of 95 participants were recruited.We also collected data on 39 HC (age = 70.3± 4.9 years, male = 27) that underwent MRI.The main inclusion criteria were mild to moderate disease stage of idiopathic Parkinson's disease, Hoehn & Yahr 2-3, age ≥ 60 years, and a MoCA score ≥ 21.People with claustrophobia or MRI incompatible implants were excluded.For further details, please see the study protocol 40 .

Demographic and clinical characteristics
Information on age, years of education, disease duration, and medication intake were collected.Motor function and balance were assessed with the MDS-UPDRS 41 and Mini-BESTest 42 , respectively.As part of the eligibility screening, MoCA 43 was used as a measure of global cognition.Self-reported balance and gait ability were captured using the Activities-specific Balance Confidence scale (ABC) 44 and Walk-12 45 , whereas anxiety and depression were assessed using the HADS 46 .

Parkinson's disease cognitive classification
Participants underwent a neuropsychological test battery comprising the following domains: executive function, attention/working memory, episodic memory, and visuospatial functions.For executive function, the Color-Word Interference Test (CWIT) and Verbal Fluency from Delis Kaplan Executive Function System (D-KEFS) 47 were used.For attention/working memory, digit span from Wechsler Adult Intelligence Scale (WAIS)-fourth edition 48 and Trail Making Test (TMT), Trials I-IV, from D-KEFS 47 , were used.For episodic memory, Rey Auditory Verbal Learning Test (RAVLT) 49 and Brief Visuospatial Memory Test-Revised (BVMT-R) 50 were performed.Finally, for visuospatial functions, the Copy condition from BVMT-R was used.
PD-MCI was calculated according to the Movement Disorder Society task force level II category 5 .Results on two neuropsychological tests within each of the aforementioned domains were compared to normative values.Trial IV from CWIT and trial II from verbal fluency (semantic fluency) were used for the executive function domain, digit span (total score), and trial IV of TMT for attention/working memory domain, delayed recall from both RAVLT and BVMT-R for episodic memory domain and copy condition from BVMT-R and the wire cube subtest from MoCA for the visuospatial domain.For the purpose of the classification, the naming and sentence subtests from MoCA were used in order to add the language domain.Depending on their performance in each of the 10 test measures (based on standard deviation (SD) from normative means), participants were given a score between 1 and 4. Cutoffs for each score were: 1 = ≤ 1 SD, 2 = 1.01 to 1.49 SD, 3 = 1.50 to 1.99 SD, and 4 = ≥ 2 SD.If a participant scored 3 or 4 on ≥ 2 tests they were classified as PD-MCI.If a participant scored 1 on all tests or had a maximum of one test scored as 2, 3, or 4, they were classified as PD-NC.Participants who scored 2 on ≥ 2 tests or score 2 on one test and 3 or 4 on one test, were put in an indeterminant group and excluded from further analyses.
To group PD-SCI participants, item 1 from the MDS-UPDRS-I was used as a criterion since it is the most common in the literature 6 .Participants scoring "1: slight" on item 1 ("1: Slight: Impairment appreciated by patient or caregiver with no concrete interference with the patient's ability to carry out normal activities and social interactions."), but those who are not classified as PD-MCI were deemed as PD-SCI.

Figure 1 .
Figure 1.Variable effect plot of dual-tasking variables over Parkinson's disease cognitive status corrected for effects of age, sex, and MDS-UPDRS-III.The colored dot shows the beta value while the whiskers are the 95% confidence interval.The gray dots display the observed values with the effects of all the control variables accounted for.The raw values were measured as dual-task step-time variability in ms and dual-task interference step-time variability in %.DTI = Dual-task interference, MDS-UPDRS-III = Movement disorder society unified parkinson's disease rating scale, MCI = Parkinson's disease with mild cognitive impairment, NC = Parkinson's disease with normal cognition, SCI = Parkinson's disease with subjective cognitive impairment. ).

Figure 2 .
Figure 2. Boxplot of Region-of-Interests (first Eigenvariate) stratified over the Parkinson's disease cognitive status.P-values annotated for significant differences.The gray dots display the observed values.DLPFC = Dorsolateral prefrontal cortex, MCI = Parkinson's disease with mild cognitive impairment, NC = Parkinson's disease with normal cognition, NBM = Nucleus basalis of meynert, SCI = Parkinson's disease with subjective cognitive impairment.

Figure 3 .
Figure 3. Interaction plot of linear regression models with Region-of-Interest (ROI) as variables.Partial effects plot corrected for age, sex, and MDS-UPDRS-III.The colorful dots display the corrected individual values.The colored shadow displays the 95% confidence interval for the respective group.The dotted lines represent the linear regression lines for each group respectively.ROIs were extracted as the first Eigenvariate and TUG dual-task measured in seconds.MDS-UPDRS-III = Movement disorder society unified Parkinson's disease rating scale, MCI = Parkinson's disease with mild cognitive impairment, NC = Parkinson's disease with normal cognition, SCI = Parkinson's disease with subjective cognitive impairment, TUG = Timed up & go test.

Figure 4 .
Figure 4. Gray matter volume differences between people with PD and healthy controls (HC).Voxel-based morphometry analyses were corrected with voxel-level p < 0.001.Images are shown in neurological convention in the MNI space (the left is on the left).The cut-out shows the NBM Region-of-Interest comparison between PD-MCI and HC.HC = Healthy controls, PD = Parkinson's disease, MCI = Mild cognitive impairment, NBM = Nucleus basalis of meynert, SCI = Subjective cognitive impairment. https://doi.org/10.1038/s41598-024-60050-5

Figure 5 .
Figure 5. Flow of building the linear regression models.To test if the effect of certain variables on the dualtask measure was different for different PD cognitive classifications (interact with PD cognitive classification), we compared with likelihood ratio tests adding those as interaction terms or additive covariates.For the ROI models, likelihood ratio tests were computed to compare models including the interaction of age and ROI.MDS-UPDRS-III = Movement disorder society unified parkinson's disease rating scale, PD = Parkinson's disease, ROI = Region-of-interest, TUG = Timed up & go test.

Table 1 .
Demographics of the participants with Parkinson's disease (PD) stratified into cognitive classifications. 1 Kruskal-Wallis-test. 2 Chi-squared test.ABC Activities-specific balance confidence scale, CI Confidence interval, DTI Dual-task intereference, HADS Hospital anxiety and depression scale, LEDD Levodopa equivalent daily dosis, MDS-UPDRS Movement disorder society unified parkinson's disease rating scale, MiniBEST Mini balance evaluation systems test, MoCA Montreal cognitive assessment, TUG Timed up & go test, yrs years.

Table 2 .
Summary of the linear regression and ANOVA models.*significant after correction for multiple comparisons.† Trend/ significant before correction for multiple comparisons.ANOVA Analysis of variance of the linear regression model, Cereb Cerebellum, DLPFC Dorsolateral prefrontal cortex, DTI Dual-task interference, Hippo Hippocampus, LR Linear regression, MDS-UPDRS-III Movement disorder society unified parkinson's disease rating scale, NBM Nucleus basalis of meynart, n.s.Not significant, n.a.Not applicable (likelihood ratio tests were not significant for the interaction term), PD Parkinson's disease, ROI Region-ofinterest, TUG Timed up & go test.